Minutes of 3rd working group meeting, Ottawa, May 18, 2001

The meeting was held at Natural Resources, Canada, upon invitation of the Geodetic Survey Division, National Resources Canada. The meeting was split in two parts:

A technical session with presentations of new developments in national or major projects, and an Arctic Gravity Project (ArcGP) business meeting where the status and conclusion of ArcGP was discussed.

Background of ArcGP

ArcGP is a working group under the International Association of Geodesy, International Gravity and Geoid Commission. The WG has the goal to compile a uniform gravity grid of the Arctic region (north of 64° N), with the aim to

Compute a uniform geoid model across the Arctic,

Provide basic data on regional geology and geophysics,

Provide data to complement satellite missions to map the gravity field, such as the ESA GOCE mission.

Complete coverage for future Earth Gravity Models.

ArcGP was initiated in 1998, and earlier WG meetings have been held in Birmingham (1999) and St. Petersburg (2000). All major countries and research institutions collecting data in the Arctic region have at present to a varying degree contributed data for the project.

Summary of the technical session û "Arctic Gravity and Geophysics"

The session was part of the Canadian Geophysical Union annual meeting. The following papers were presented:

R. Forsberg (KMS, Denmark; chairman of ArcGP): Arctic Gravity Project, status and background. Gave an overview of the project history and current status. Also honored the late Sergei S. Maschenkov (VNIIO), who passed away this winter, for his influential role in the establishment and cooperation of ArcGP.

B. Coakley (Tulane Univ., USA): Gravity and bathymetry of the SCICEX submarine program. Outlined highlights of the 1999 Scicex cruise, such as the detection of active lava flows on the Gakkel Ridge, and new data from Lomonossov Ridge, indicating an ice cap existed there during last glacial maximum. Described processing status of 1999 gravity data.

V. Childers, J. Brozena (Naval Research Lab., USA): NRL long-range airborne Arctic geophysics program û an update. NRL have covered major parts of the central Arctic Ocean with long-range aerogravity, at accuracy of 2 mGal and resolution 15 km. No new arctic data have been collected since St. Petersburg meeting.

D. Solheim, A. Gidskehaug (Norway): Svalbard and Fram Strait airborne gravity activities 1998-2001. Described airborne gravity surveys around the Svalbard archipelago and surrounding seas, carried out in close cooperation with KMS. Presented some problems with downward continuation over land areas with glaciers. Some 1999 data still to be processed.

G. Deminov, A. Mayorov (Tsniigaik, Russia): The problem of world height system establishment by gravity and GPS data û example of Arctic geoid. Stressed the need for computing quasigeoid to link vertical systems and establish a unified global vertical datum. ArcGP geoid aspects very important. Outlined ongoing Russian program with GPS on benchmarks.

NGA, R. Forsberg: Processing of ArcGP gravity data and some intercomparisons. Gave data status of data submitted so far to ArcGP computing center at NGA, with some comparisons of the problems of data merging of data with different resolution, and presented overview gravity map. Current data coverage percentage in terms of different sources:

NRL airborne surveys + PMAP airborne + KMS airborne + Scicex: 34%

NGA point gravity sources: 18%

NOAA and KMS satellite altimetry anomalies: 14%

NRL Greenland survey 1991-92 7%

Tsniigaik European Russia 10Æ data + Scandinavia + NGA/KMS marine 10%

Eastern central arctic VNIIO data (release 1:6 mio CRDF project) 5%

EGM96 30Æ fill-in gravity anomalies 12%

Protocol of the ArcGP business meeting

Afternoon was reserved for discussions on the project, reaffirming the basic principles of data release, grid preparation, remaining problems with data, potential new data, and publication policy. The meeting had simultaneous translation from/to Russian.

Basic principles of data exchange and project progress

The participants reaffirmed the following principles:

Only 5Æ gridded data to be provided. Raw data will not be made available. Grid should represent point values at the 5Æ nodes.

Derived products (geoid, Bouguer anomaly) to be released along with basic grid.

Russian institutions includes in the Project only data open according to the Russian legislation.

Final product of the Project (digital grid) will be distributed among participant without any restrictions, and later released into general public.

The first phase of the project to be ended fall of this year, with grid preparation by NGA/KMS and distribution following final compilation of "beta-version" grid.

GUNIO/HDNO Russia invited to participate in the Project at any stage (including consideration of possible production of the final map in their mapping department), ref. offer at St. Petersburg workshop.

Time schedule

Conclusion of first phase (beta version grid release): end of year 2001. To get a more specific time table for grid preparation etc. (we failed to make a clear agreement here), I suggest:

Deadline for data contribution: ultimo November 2000.

Grid preparation and publishing: February 2002.

There is some time-pressure to publish grid soon, as NASAÆs laser satellite ICESAT will provide altimetry over entire Arctic Ocean by 2002, and thus may make major parts of ArcGP grid outdated.

Grid preparation

The following principles were confirmed: The method of least-squares collocation/Kriging is an efficient way to combine data with different resolutions and accuracy, especially when combined with draping of noisy data with bias problems on good data. Some bias problems were pointed out, e.g. apparent base tie problems in some Scicex cruises. Russian data to be converted from state reference system to international gravity datum for optimal geoid determination results, using conversion formula provided by Tsniigaik.

Important to provide some kind of standard deviation/data quality indication along with grid and geoid. Important to avoid data-coverage "artifacts" in final grid (such as e.g. discontinuities along meridians or parallels).

The participants were asked to look for more potential data. It was requested that data as far as possible be contributed during the fall 2001. The participants acknowledged the data contributions released by all countries, especially the major contributions by NGA, NRL and the Scicex program, as well as the potential important contributions from Russia (pending approval of release policy by central authorities). Considering the monumental effort of Russian gravity surveys in the Arctic over decades, the participants recommended that such data be made available to ArcGP at map scales comparable to the general ArcGP data resolution, to result in a high-quality, truly international ArcGP grid and geoid product.

Publication and dissemination

It was suggested to make primary distribution of ArcGP results by web and CD-ROM. A printed map is to be compiled, preferably at same scale as the IBCAO international bathymetry map of the Arctic Ocean. Agency to print this map still to be decided. An EOS article planned, with all major data providers as co-authors, to highlight completion of project, followed by a detailed technical report describing grid preparation etc. Geoid computation issues are to be especially addressed in Journal of Geodesy paper. Reports at e.g. EGS, AGU and IUGG conferences by members of the working group should be done to enhance visibility of results.

It was decided that momentum on project is now for completion, and no further working group meeting seems necessary at present.

The participants thanked Marc Veronneau of GSD/NRCan for arranging the meeting, and for holding the ArcGP evening dinner.

Copenhagen, May 29, 2001

Rene Forsberg

Chairman of Arctic Gravity Project

State Geodesist, National Survey and Cadastre (KMS), Denmark (rf@kms.dk)

Participants in ArcGP workshop and business meeting, Ottawa, Canada, May 2001